The present invention relates to spread-spectrum, Code Division Multiple Access (CDMA) communication systems. More particularly, and not by way of limitation, the present invention is directed to a joint detector for receiving an Orthogonal Variable Spreading Factor (OVSF) code-sharing, low-rate, code-multiplexed channel. The OVSF code, often referred to as the channelization code or spreading code, is used to spread a lower data rate signal to the transmission baud (chip) rate, which may be 3.84 mega-baud (or chip) per second according to the Wideband CDMA (WCDMA) standard, or 1.2288 mega-baud (or chip) per second according to the IS-95 and cdma2000 standards.
In WCDMA, an enhancement known as Enhanced Uplink is contemplated for reducing delays, improving uplink high-data-rate coverage, and increasing capacity. A key enabler for meeting these objectives is Hybrid Automatic Repeat Request (HARQ) with fast retransmission and soft combining. To support uplink HARQ operations, Enhanced HARQ Indication Channels (E-HICHs) are needed in the downlink for the base station to signal Ack or Nack messages. HARQ is of critical importance in reducing round-trip delays and improving uplink high-data-rate coverage and capacity. Therefore it is highly desirable to have reliable E-HICH reception.
In order to avoid scheduling delay, the current concept of Enhanced Uplink allows a mobile terminal to transmit in the uplink direction without a scheduling grant as long as the transmission data rate is not exceedingly high. As a result, many mobile terminals may simultaneously transmit in the uplink direction using the HARQ protocol. In response, the base station must provide many E-HICHs in a transmission time interval (TTI). To avoid having E-HICHs consume too many OVSF codes, an OVSF-code sharing, code-division multiplexed (CDM) architecture has been introduced for E-HICHs. According to this CDM approach, a number of E-HICHs share a common OVSF (channelization) code. A 1-bit Ack/Nack message of each E-HICH is modulated (spread) by a user-specific signature sequence before OVSF spreading. With this approach, the signals transmitted on the code-sharing E-HICH are mutually orthogonal through the use of mutually orthogonal Hadamard sequences as the signature sequences. According to the CDM architecture, the duration of the signature sequence is one slot. For clarity in the following description, the term “OVSF code” or “channelization code” means the spreading sequence that spreads an input signal to the WCDMA chip rate (3.84 Mcps), and the term “signature sequence” means the spreading sequence applied to the 1-bit Ack/Nack information.
FIG. 1 is a simplified block diagram illustrating the generation of E-HICHs according to the preexisting CDM architecture. As shown, E-HICHs for users 1-K share a common OVSF (channelization) code channel. Prior to the normal OVSF spreading at 11, signature sequences are employed at 121 through 12K to separate the E-HICHs sharing the same OVSF code. According to the CDM architecture in WCDMA, the common OVSF code has a spreading factor of 128, giving rise to 20 symbols in a slot. The signature sequence is thus based on length-40 Hadamard sequences and QPSK modulation mapping every two bits of the Hadamard sequence to one QPSK symbol. In this case, the different transmitted CDM E-HICHs are mutually orthogonal when the signal is integrated over a slot.
At the receiver, the orthogonality also holds if the fading channel is non-dispersive and constant within a slot. With multipath, the orthogonality remains high as long as the channel is constant within a slot due to the large processing gain (2560) against the inter-chip interference. Orthogonality, however, is very much compromised in high Doppler channels in which multipath fading varies noticeably in the slot interval. Conventionally, the received signal is first RAKE processed (i.e., despread and combined using the common OVSF code), and the RAKE receiver output is correlated with the Hadamard sequence associated with the E-HICH of interest. In a time-varying channel, the orthogonality between the various E-HICHs sharing the same OVSF code cannot be preserved when simply correlating with the signature sequence of interest. Loss of orthogonality results in co-channel interference.
FIG. 2 is a simplified block diagram illustrating the transmission of E-HICHs to two mobile terminals 15 and 16 in a near-far scenario. Loss of orthogonality may result in severe performance degradation when the base station 17 uses a relatively weak transmit power to transmit a signal 18 to the near-end mobile terminal 15 and a much stronger transmit power to transmit a signal 19 to the far-end mobile terminal 16. In this case, the E-HICH signal intended for the far-end mobile terminal can cause significant interference at the near-end mobile terminal's receiver, resulting in significant degradation to the near-end mobile terminal's E-HICH performance.